Apparatus for producing crystalline semiconductor material



Get. 19, 1965 w SMITH 3,212,858

APPARATUS FOR PRODUCING CRYSTALLINE SEMICONDUCTOR MATERIAL Filed Jan.28, 1963 United States Patent 3,212,858 APPARATUS FOR PRODUCINGCRYSTALLINE SEMICONDUCTOR MATERIAL Walter J. Smith, Pitcairn, Donald R.Hamilton, Monroeville, and Jacob J. Coleman, Braddock, Pa., assignors toWestinghouse Electric Corporation, Pittsburgh, Pa., a corporation ofPennsylvania Filed Jan. 28, 1963, Ser. No. 254,233 2 Claims. (Cl.23-273) This invention relates to apparatus for growing ribbons andother shapes of crystalline semiconductor material from a melt in aheated crucible.

Conventional apparatus is available for producing a solid crystal from amolten pool of suitable material. Such apparatus includes means forcontacting the surface of the melt with a previously prepared crystal orseed and then slowly withdrawing the seed. The seed pulls after itself aportion of the melt which solidifies on the seed, so a crystal ofindefinite length can be produced. In growing crystals the thermalgradient at the surface of the melt is important. Unless it iscontrolled properly the result can be the growth of thick, roughcrystals or even no growth at all. Heretofore, the melt has beencontained in an uncovered crucible. The melt surface radiates heat at arate depending on its emissivity and temperature, and the thermalgradient at the melt surface and just within the melt will be equal tothe radiation loss per unit area divided by the thermal conductivity ofthe melt. In many cases, such as in the growth of silicon dendrites,this thermal gradient is unsuitably high, so that when dendrites ofgreat length are grown, they deteriorate in time to an unsatisfactorycondition. That is, they become thick, rough and have macroscopic pitsand hillocks on their surfaces.

It is among the objects of this invention to provide apparatus forproducing crystalline semiconductor material that does not deterioratematerially, that can be formed in indefinite lengths, and that retainsits original thickness and smoothness. A further object is to providesuch apparatus, in which supercooling of the melt is controlled.

The invention is illustrated in the accompanying drawings, in which:

FIG. 1 is a side view of apparatus constructed for our purposes;

FIG. 2 is a reduced plan view of the supercooling control member on thecrucible; and

FIG. 3 is a plan view of a modification.

Referring to FIG. 1 of the drawings, a crucible 1 of well-knownconstruction for growing dendrites and other crystalline semiconductormaterials is made from graphite or the like and may be mounted on acenter post 2 projecting up from a base member 3. The upper part of thecrucible is provided with an upwardly opening recess 4 to contain thematerial 5 from which a crystal is to be formed. The crucible is mountedin a furnace of any suitable construction, the one shown being acylindrical furnace 6 formed from quartz or the like to provide amplevisibility into its interior. The top of the furnace is provided with asmall central opening 7, from which a tube 8 extends upwardly. The upperend of the tube is closed by a cover 9 that has a central opening 10just large enough to receive a rod 11 or other suitable elongated memberto which a suitable seed is attached and by which a crystal can be drawnfrom the crucible. The side wall of the furnace is provided with anopening 12 connected to a pipe 13, through which an inert gas can bedelivered to the inside of the furnace or the air exhausted from thefurnace.

The crucible is heated most suitably by a radio frequency coil 15encircling it. This coil may be either inside or outside of the furnace,outside being preferred. The turns of the coil are closer together atthe bottom of the crucible than at its top so that there will be athermal gradient present in the crucible-melt system to insure that thesurface of the melt and the melt just below its surface will be thecoolest points of the melt. Also, this thermal gradient maintains thebottom part of the melt, which is in contact with the crucible, at atemperature higher than the melting point of the melt, whereby the meltis prevented from freezing at the bottom.

Another feature of this invention isthat the desired thermal conditionsfor continued crytsal growth are produced by reflecting some of the heatfrom the surface of the melt back to that surface in order to reduce thethermal gradient in the melt and thereby control supercooling of the topof the melt. This is accomplished by a supercooling control member 17that is placed on top of the crucible to cover it and hold in the heat,whereby to provide as large a supercooled area of the melt around thegrowing crystal as is possible. To permit the growing crystal to beremoved from the crucible, the top of the control member is providedwith a center opening 18 for rod 11. This opening should be as small aspossible but, as shown in FIG. 2, it is preferred to make it in the formof a slot extending part way across the control member so that anobserver can see down into the crucible through either end of the slot.This control member 17, which does not interfere with continuousextraction of the crystalline material, controls supercooling of moltenmaterial in the crucible by reducing the thermal gradient at the top ofthe pool. For best results, it also is desirable to heat the controlmember, and this can be done by extending the heating coil around it asshown.

In the modification shown in FIG. 3, the central opening 20 in thesupercooling control member 21 to permit withdrawing the crystal fromthe crucible is small and circular, but the control member also isprovided with a pair of arcuate slots 22 spaced uniformly from itscenter and opening into the crucible. These slots not only provide sightopenings through the control member, but they allow more heat to beradiated from the area of the melt beneath the slots in comparison withthe area of the melt directly beneath the unslotted portion of thecontrol member. This results in a higher temperature at the edge of thecrucible for the same supercooling at the center, thus decreasing thetendency for the melt to freeze at the melt-crucible interface.

For a more detailed explanation of the process of growing dendriticcrystals from a supercooled melt reference should be had to U.S. Patent3,031,403, the assignee of which is the same as that of the presentinvention.

According to the provisions of the patent statutes, we have explainedthe principle of our invention and have illustrated and described whatwe now consider to represent its best embodiment. However, we desire tohave it understood that the invention may be practiced otherwise than asspecifically illustrated and described.

We claim:

1. In a furnace for the preparation of crystalline semiconductormaterial from a supercooled melt, a crucible for containing the melt, amember disposed over and covering the top of said crucible, said memberhaving a central opening and arcuate openings spaced uniformly aboutsaid central opening, and a heating coil disposed about said crucible,said heating coil having its upper turns spaced farther apart than itslower turns.

2. In a furnace for the preparation of crystalline semiconductormaterial from a supercooled melt, a crucible for containing the melt, amember disposed over and covering the top of said crucible, said memberhaving a central opening through which a seed is passed to contact themelt, arcuate openings in said member, said arcuate members beingdisposed uniformly about said central opening, said arcuate membersserving to control the thermal gradients near the surface of the meltand a heating coil disposed about said crucible, said heating coilhaving its upper turns spaced farther apart than its lower turns.

References Cited by the Examiner UNITED STATES PATENTS 2,809,136 10/57Mortimer 23- 301x 2,822,308 2/58 Hall.

5 2,956,863 10/60 Goorissin 23-273 X 3,031,403 4/62 Bennet 23 301 XOTHER REFERENCES Design of Laboratory Furnaces by Start et al., Journal10 of Sci. Ins., vol. 37, January 1960, pages 1'7 to 24.

NORMAN YUDKOFF, Primary Examiner.

1. IN A FURNACE FOR THE PREPARATON OF CRYSTALLINE SEMICONDUCTOR MATERIALFROM A SUPERCOOLED MELT, A CRUCIBLE FOR CONTAINING THE MELT, A MEMBERDISPOSED OVER AND COVERING THE TOP OF SAID CRUCIBLE, SAID MEMBER HAVINGA CENTRAL OPENING AND ARCUATE OPENINGS SPACED UNIFORMLY ABOUT SAIDCENTRAL OPENING, AND A HEATING COIL DISPOSED ABOUT SAID CRUCIBLE, SAIDHEATING COIL HAVING ITS UPPER TURNS SPACED FARTHER APART THAN ITS LOWERTURNS.